Ceramic Materials For 4-Way And NOx Adsorber And Method For Making Same

ABSTRACT

Disclosed are ceramic articles comprising a sintered phase ceramic composition containing, as expressed on a weight percent oxide basis: a(Al 2 TiO 5 )+b(ZrTiO 4 )+c(Y 2 O 3 )+d(YPO 4 ) wherein “a, b, c, and d” represent weight fractions of each component such that (a+b+c+d)=1.00 and wherein 0.5&lt;a≦0.95; 0≦b≦0.5, 0.0≦c≦0.10, and 0≦d≦0.5. Also disclosed are precursor batch compositions and methods for manufacturing the ceramic articles disclosed herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to ceramic articles, and more particularlyto ceramic articles having properties suitable for use in exhaustafter-treatment applications, particularly diesel exhaust filtration.

2. Technical Background

Recently, much interest has been directed towards the diesel engine dueto its efficiency, durability, and economical aspects. However, dieselemissions have been scrutinized both in the United States and Europe. Assuch, stricter environmental regulations will require diesel engines tobe held to the similar standards as gasoline engines. Therefore, dieselengine manufacturers and emission-control companies are working toachieve a diesel engine which is efficient, cleaner and meets the moststringent of requirements under all operating conditions with minimalcost to the consumer.

Tighter regulations continue to drive the improvement of after-treatmenttechnologies for the abatement of diesel emissions. A 4-way emissionreduction system (which simultaneously control HC, NOx, CO and PMemissions) is the future technology desired by many original equipmentmanufacturers. Current 4-way emission reduction technology propose useof a NOx adsorber (storage) material coated on a fine porous ceramicfilter. In use, the engine cycles between rich and lean operation modes.During lean engine operation, particulate matter is oxidized by activeoxygen released from the NOx adsorbers and excess oxygen from theexhaust. During the rich mode, NOx is reduced while particulate matteris also oxidized by active oxygen released during the adsorbed NOxreduction.

Cordierite is presently being proposed as a porous substrate materialfor NOx adsorbing emission reduction systems, however its use has beenlimited to only barium-based NOx adsorbing catalyst formulations.However, NOx slippage with barium-based applications is a concern, assuch slippage may occur during uncontrolled exotherms exceeding 600° C.In contrast, NOx adsorber based upon a potassium (K) catalyst would behighly desired by engine manufacturers because it may offer a broaderand higher operable temperature window for NOx reduction activity.Additionally, potassium (K) is also preferable as it is not regulated astoxic heavy metal that is detrimental to the environment and humanhealth.

However, cordierite, a primary ceramic material for present aftertreatment systems, contains silicate phases that are prone to react withpotassium-based NOx adsorber washcoats used in diesel particulate NOxreduction (DPNR) systems. To this end, silicates within currentcordierite ceramic substrate technology have been shown to leach andreact with potassium based catalysts to form potassium silicate. Thisleaching reaction compromises both the durability of the cordieritefilter substrate and NOx absorption function of the potassium washcoat.For this reason, cordierite is considered to be incompatible with apotassium-based DPNR system. Further, although operating temperatures ofcurrent DPNR systems are typically below 800° C. due to the presence ofthe catalyst, the low heat capacity of cordierite can present additionalconcerns. In particular, current DPNR systems typically require aceramic support substrate having a porosity of at least 50%. Thisrelatively high porosity reduces the heat capacity of the cordieritesubstrate and decreases thermal mass, thereby increasing the likelihoodof a possible temperature excursion leading to a failure during use.

Accordingly, there is a need in the exhaust after treatment art for adurable potassium-resistant ceramic article having excellent thermalshock resistance, high heat capacity, and refractory character, that issuitable for use as a catalyst support substrate in diesel exhaust gastreatment and, in particular, in 4-way and NOx diesel exhausts treatmentapplications.

SUMMARY OF THE INVENTION

The present invention relates to ceramic articles, and more particularlyto ceramic articles having properties suitable for use in systemsemploying 4-way and NOx adsorber technologies, such as in exhaust aftertreatment applications. The ceramic articles can be formed of a porousnon-silicate ceramic oxide capable of supporting a catalyst formulation.

In one aspect, a ceramic article according to the present inventioncomprises a sintered phase ceramic composition containing, as expressedon a weight percent oxide basis: a(Al₂TiO₅)+b(ZrTiO₄)+c(Y₂O₃)+d(YPO₄)wherein a, b, c, and d are weight fractions of each component such that(a+b+c+d)=1.00 and wherein 0.5<a≦0.95; 0≦b≦0.5, 0.0≦c≦0.10, and 0≦d≦0.5.The ceramic articles can further comprise an alkali-based or alkalineearth-based catalyst formulation, including for example, apotassium-based catalyst formulation. The absence of silica in theceramic article can, in one aspect, enable the ceramic articles to beresistant to potassium migration. In still another aspect, the ceramicarticles of the present invention exhibit relatively high heat capacitywith relatively low coefficients of thermal expansion. When catalyzed,the resulting ceramic body remains durable; maintaining its low CTE andhigh thermal shock resistance.

The present invention also provides a method for producing the ceramicarticles disclosed herein. The method generally comprises the steps ofproviding a plasticized ceramic precursor batch composition comprising:an inorganic batch composition containing an alumina source, a titaniasource, and at least one of zirconia source, a yttria source, aphosphate source, and a yttrium phosphate source, and an optionalyttrium oxide sintering aid. The plasticized ceramic precursor batchcomposition further comprises an organic binder; and a liquid vehicle.An extruded green body can be formed from the plasticized ceramicprecursor batch composition and fired under conditions effective toconvert the ceramic precursor composition into a sintered phase ceramiccomposition.

Additional aspects of the invention will be set forth, in part, in thedetailed description, figures and any claims which follow, and in partwill be derived from the detailed description, or can be learned bypractice of the invention. It is to be understood that both theforegoing general description and the following detailed description areexemplary and explanatory only and are not restrictive of the inventionas disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate certain aspects of the instantinvention and together with the description, serve to explain, withoutlimitation, the principles of the invention.

FIG. 1 depicts an electron microprobe result of a potassium washcoat ona ceramic article comprising 80% AT/20% ZT and after aging at 850° C.for 10 hours in 10% steam.

FIG. 2 depicts another example of a electron microprobe result ofpotassium washcoat on a ceramic article comprising 80% AT/20% ZT andafter aging at 850° C. for 10 hours in 10% steam.

FIG. 3 is a graph illustration showing the stability of the aluminumtitanate phase in the ceramic composition of the present invention.

FIG. 4 illustrates an examplary pressure drop curve for an inventiveceramic composition comprising 80% AT and 20% ZT.

FIG. 5 is a comparison plot showing typical effects of pressure drop ontotal porosity in the absence of a catalysts washcoat.

FIG. 6 is a scanning electron micrograph of an inventive ceramiccomposition comprising 80% AT and 20% ZT sample.

FIG. 7 is a scanning electron micrograph of an inventive ceramiccomposition comprising Yttrium phosphate and aluminum titante.

FIG. 8 is a scanning electron micrograph of an inventive ceramiccomposition comprising Yttrium phosphate and aluminum titante.

DETAILED DESCRIPTION OF THE INVENTION

The present invention can be understood more readily by reference to thefollowing detailed description, examples, and claims, and their previousand following description. However, before the present compositions,devices, and/or methods are disclosed and described, it is to beunderstood that this invention is not limited to the specific articles,devices, and/or methods disclosed unless otherwise specified, as suchcan, of course, vary. It is also to be understood that the terminologyused herein is for the purpose of describing particular aspects only andis not intended to be limiting.

The following description of the invention is provided as an enablingteaching of the invention in its best, currently known embodiment. Tothis end, those skilled in the relevant art will recognize andappreciate that many changes can be made to the various aspects of theinvention described herein, while still obtaining the beneficial resultsof the present invention. It will also be apparent that some of thedesired benefits of the present invention can be obtained by selectingsome of the features of the present invention without utilizing otherfeatures. Accordingly, those who work in the art will recognize thatmany modifications and adaptations to the present invention are possibleand can even be desirable in certain circumstances and are a part of thepresent invention. Thus, the following description is provided asillustrative of the principles of the present invention and not inlimitation thereof.

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to an “alumina source” includes aspects having two ormore such alumina sources unless the context clearly indicatesotherwise.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another aspect includes from the one particular value and/orto the other particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

As used herein, the terms “optional” or “optionally” mean that thesubsequently described event or circumstance may or may not occur, andthat the description includes instances where said event or circumstanceoccurs and instances where it does not.

As used herein, a “weight percent” or “percent by weight” of acomponent, unless specifically stated to the contrary, is based on thetotal weight of the formulation or composition in which the component isincluded.

As briefly introduced above, the present invention provides an improvedceramic articles useful for high temperature ceramic filterapplications. In one aspect, the ceramic articles are suitable for useas a “4-Way” diesel particular filter (DPF) capable of reducing one ormore of Hydrocarbons (HC), Carbon Monoxide (CO), Nitrogen Oxides (NOx),and Particulate Matter (PM) from a stream of diesel exhaust gas. To thatend, the ceramic article is comprised of a sintered phase ceramic oxidecomposition containing, as expressed on a weight percent oxide basis:a(Al₂TiO₅)+b(ZrTiO₄)+c(Y₂O₃)+d(YPO₄); wherein a, b, c, and d are weightfractions of each component such that (a+b+c+d)=1.00; and wherein0.5<a≦0.95; 0≦b≦0.5, 0.0≦c≦0.10, and 0≦d≦0.5.

As indicated, the sintered phase ceramic oxide composition comprisesaluminum titanate (Al₂TiO₅) in an amount “a”, expressed on a weightpercent oxide basis as a weight fraction of the oxide components,wherein 0.5<a≦0.95. Thus, it should be understood that the weightfraction “a” of aluminum titanate can be any value within the statedrange, including for example, 0.6, 0.7, 0.8 and 0.9. Still further, theweight fraction “a” of aluminum titanate can also be within any range ofvalues derived therefrom, including for example 0.6<a≦0.8.

The ceramic oxide composition can optionally comprise zirconium titanate(ZrTiO₄) in an amount “b”, expressed on a weight percent oxide basis asa weight fraction of the oxide components, wherein 0≦b≦0.5. Thus, itshould be understood that the weight fraction “b” of zirconium titanatecan be any value within the stated range, including for example, 0.1,0.2, 0.3 and 0.4. Still further, the weight fraction “b” of zirconiumtitanate can also be within any range of values derived therefrom,including for example 0.20≦b≦0.40.

The ceramic oxide composition can also optionally comprise yttrium oxide(Y₂O₃). When present, the yttrium oxide can in one aspect be utilized asa sintering aid during manufacture of the sintered phase composition. Tothat end, as summarized above, the yttrium oxide is optionally presentin an amount “c”, expressed on a weight percent oxide basis as a weightfraction of the oxide components, wherein 0.0≦c≦0.10. Thus, it should beunderstood that the weight fraction “c” of yttrium oxide can be anyvalue within the stated range, including for example, 0.01, 0.03, 0.05,0.07, and 0.09. Still further, the weight fraction “c” of yttrium oxidecan also be within any range of values derived therefrom, including forexample 0.0≦c≦0.05.

Still further, the ceramic oxide composition further optionallycomprises yttrium phosphate (YPO₄ and also known as xenotime) in anamount “d”, expressed on a weight percent oxide basis as a weightfraction of the oxide components, wherein 0≦d≦0.5. Thus, it should beunderstood that the weight fraction “d” of yttrium phosphate can be anyvalue within the stated range, including for example, 0.05, 0.15, 0.25,0.35, and 0.45. Still further, the weight fraction “d” of yttriumphosphate can also be within any range of values derived therefrom,including for example 0.05≦d≦0.5.

The ceramic oxide composition can comprise aluminum titanate togetherwith any one or more of the zirconium titanate, yttrium oxide, andyttrium phosphate. For example, in an exemplary aspect, the sinteredphase ceramic oxide composition can consist essentially of aluminumtitanate, zirconium titanate, and yttrium oxide, wherein the respectiveweight fractions of the aluminum titanate, zirconium titanate, andyttrium oxide are: 0.60≦a≦0.80 and 0.20≦b≦0.40 and 0.00≦c≦0.05. In stillanother exemplary aspect, the sintered phase ceramic oxide compositioncan consist essentially of aluminum titanate and yttrium phosphate,wherein the respective weight fractions of the aluminum titanate and theyttrium phosphate are: 0.5≦a≦0.95 and 0.05≦d≦0.5.

The sintered phase ceramic oxide composition can further becharacterized by its total porosity in %, as measured by mercuryporosimetry, as being relatively high. In one aspect, the total porosityof the ceramic oxide composition is at least 40%. In another aspect, thetotal porosity % can be at least 45%, at least 50%, at least 55%, oreven at least 60%. In still another aspect, the total porosity % may bein any range derived from these exemplary porosity values, including forexample, in the range of from 40% to 50%, from 45% to 55%, or even from50% to 60%.

Another advantage of the inventive ceramic article is a relatively lowthermal expansion, which can result in excellent thermal shockresistance (TSR). TSR is inversely proportional to the coefficient ofthermal expansion (CTE). That is, a ceramic article having relativelylow thermal expansion will exhibit good thermal shock resistance and cansurvive the wide temperature fluctuations that are encountered in enduse filter applications. Accordingly, in one aspect, the ceramicarticles of the present invention are characterized by having arelatively low coefficient of thermal expansion (CTE), measured bydilatometry, in at least one direction, i.e., the axial and/ortransverse direction, wherein CTE is less than or equal to 20.0×10⁻⁷/°C. across the temperature range of from 25° C. to 1000° C. In anotheraspect, the CTE is less than or equal to 15.0×10⁻⁷/° C. across thetemperature range of from 25° C. to 1000° C. Still further, in anotheraspect, the CTE of the ceramic article is less than or equal to12.0×10⁻⁷/° C. across the temperature range of from 25° C. to 1000° C.In still another aspect, the CTE is less than or equal to 10.0×10⁻⁷/° C.across the temperature range of from 25° C. to 1000° C. In anotheraspect, the CTE does not change appreciably after aging in 10% steam at800° C. for 65 hours. In particular, the CTE is less than 20.0×10⁻⁷/° C.after such aging in 10% steam at 800° C. for 65 hours.

The ceramic bodies of the present invention can also comprise arelatively narrow pore size distribution evidenced by a minimizedpercentage of relatively fine and/or relatively large pore sizes. Tothis end, relative pore size distributions can be expressed by a porefraction which, as used herein, is the percent by volume of porosity, asmeasured by mercury porosimetry, divided by 100. For example, thequantity d₅₀ is the median pore size based upon pore volume, and ismeasured in micrometers; thus, d₅₀ is the pore diameter at which 50% ofthe open porosity of the ceramic has been intruded by mercury. Thequantity d₉₀ is the pore diameter at which 90% of the pore volume iscomprised of pores whose diameters are smaller than the value of d₉₀;thus, d₉₀ is also equal to the pore diameter at which 10% by volume ofthe open porosity of the ceramic has been intruded by mercury. Stillfurther, the quantity d₁₀ is the pore diameter at which 10% of the porevolume is comprised of pores whose diameters are smaller than the valueof d₁₀; thus, d₁₀ is equal to the pore diameter at which 90% by volumeof the open porosity of the ceramic has been intruded by mercury.

The median pore diameter or median pore size (MPS), d₅₀, of the porespresent in the instant ceramic articles can, in one aspect, be greaterthan 1 μm, greater than 5 μm, greater than 10 μm, greater than 15 μm, oreven greater than 20 μm. In another aspect, the median pore diameter canbe in the range of from 1 μm to 25 μm. In still another aspect, themedian pored diameter can be in the range of from 5 μm to 15 μm.

The ceramic articles of the present invention can have any shape orgeometry suitable for a particular application. In high temperaturefiltration applications, such as diesel particulate filtration, forwhich the inventive bodies are in one aspect especially well suited, itis preferred for the bodies to have a multicellular structure, such asthat of a honeycomb monolith. An exemplary honeycomb structure can havean inlet and outlet end or face, and a multiplicity of cells extendingfrom the inlet end to the outlet end, the cells having porous walls. Thehoneycomb structure can further have cellular densities from 70cells/in² (10.9 cells/cm²) to 400 cells/in² (62 cells/cm²).

A portion of the cells at the inlet end or face end can, in one aspect,be plugged with a paste having same or similar composition to that ofthe honeycomb structure, as described in U.S. Pat. No. 4,329,162 whichis herein incorporated by reference. The plugging is only at the ends ofthe cells which is typically to a depth of about 5 to 20 mm, althoughthis can vary. A portion of the cells on the outlet end but notcorresponding to those on the inlet end are plugged. Therefore, eachcell is plugged only at one end. A preferred arrangement is to haveevery other cell on a given face plugged as in a checkered pattern. Thisplugging configuration allows for more intimate contact between theexhaust stream and the porous wall of the substrate. The exhaust streamflows into the substrate through the open cells at the inlet end, thenthrough the porous cell walls, and out of the structure through the opencells at the outlet end. Filters of the type herein described are knownas “wall flow” filters since the flow paths resulting from alternatechannel plugging require the exhaust being treated to flow through theporous ceramic cell walls prior to exiting the filter.

The ceramic articles of the present invention can further comprise acatalyst washcoating applied thereon. As one of ordinary skill in theart will appreciate, the particular catalyst washcoating desired will bedependent upon the particular species to be adsorbed and/or filteredfrom an exhaust stream. However, in an exemplary aspect wherein theceramic article is for adsorbing and reducing one or more of Hydrocarbon(HC), Carbon Monoxide (CO), Nitrogen Oxides (NOx), and ParticulateMatter (PM) from a stream of diesel exhaust gas, the catalystwashcoating can comprise an alkali metal or alkali earth metal catalyst,including for example, barium, strontium, and/or potassium.

The present invention also provides a method for manufacturing ceramicarticles as described herein. Any high temperature ceramic processingmethods such as reaction-sintering or melting of inorganic batchprecursors can be used. In one aspect, the method according to thepresent invention generally comprises the steps of first providing aplasticized ceramic precursor batch composition comprising an inorganicceramic forming batch composition; an organic binder; and a liquidvehicle. An extruded green body can then be formed from the plasticizedceramic precursor batch composition. Once formed, the extruded greenbody can be fired under conditions effective to convert the ceramicprecursor composition into a sintered phase ceramic composition.

The inorganic ceramic forming batch composition comprises an aluminasource and a titania source. Depending on the desired composition of theresulting sintered phase ceramic oxide composition, the inorganicceramic forming batch composition further comprises at least one of azirconia source, a phosphorous oxide source, a yttria source, andyttrium phosphate source, and an optional yttrium oxide sintering aid.For example, in one exemplary aspect, the inorganic ceramic formingbatch composition comprises an alumina source, a titania source, and azirconia source. In another exemplary aspect, the inorganic ceramicforming batch composition further comprises an alumina source, a titaniasource, and a yttrium phosphate source.

The inorganic ceramic batch components can also be naturally occurringand/or synthetically produced materials including components in the formof carbonates, phosphates, nitrates, citrates, sulfides, hydrates,oxides, and the like. Thus, it should be understood that the presentinvention is not limited to any particular types of powders or rawmaterials, as such can be selected depending on the properties desiredin the final ceramic body.

Exemplary alumina sources can include aluminum oxides or a compoundcontaining aluminum which when heated to sufficiently high temperatureyields essentially 100% aluminum oxide. Non-limiting examples of aluminaforming sources include corundum or alpha-alumina, gamma-alumina,transitional aluminas, aluminum hydroxide such as gibbsite and bayerite,boehmite, diaspore, aluminum isopropoxide and the like. Commerciallyavailable alumina sources can include relatively coarse aluminas, suchas the Alcan C-700 series, having a particle size of about 4-6micrometers, and a surface area of about 0.5-1 m²/g, e.g., C-714 andC-701™. Relatively fine aluminas, having a particle size of about 0.5-2micrometers, and a surface area of about 8-11 m²/g, can also be used,such as A10 and A-16SG alumina, available from Alcoa. In still anotheraspect, the alumina forming source can be a colloidal alumina havingparticles sizes in the range of from 20 nm to 50 nm, such as the AL20available from Nyacol.

If desired, the alumina source can also comprise a dispersible aluminaforming source. As used herein, a dispersible alumina forming source isan alumina forming source that is at least substantially dispersible ina solvent or liquid medium and that can be used to provide a colloidalsuspension in a solvent or liquid medium. In one aspect, a dispersiblealumina source can be a relatively high surface area alumina sourcehaving a specific surface area of at least 20 m²/g. Alternatively, adispersible alumina source can have a specific surface area of at least50 m²/g. In an exemplary aspect, a suitable dispersible alumina sourcefor use in the methods of the instant invention comprises alpha aluminumoxide hydroxide (AlOOH.×.H₂O) commonly referred to as boehmite,pseudoboehmite, and as aluminum monohydrate. In another exemplaryaspect, the dispersible alumina source can comprise the so-calledtransition or activated aluminas (i.e., aluminum oxyhydroxide and chi,eta, rho, iota, kappa, gamma, delta, and theta alumina) which cancontain various amounts of chemically bound water or hydroxylfunctionalities. Specific examples of commercially available dispersiblealumina sources that can be used in the present invention include,without limitation, Dispal Boehmite, commercially available from CONDEAVista Company of Houston, Tex., and Alpha Alumina A1000, commerciallyavailable from Almatis, Inc.

The titania source can in one aspect be a titanium dioxide composition,such as rutile titania, anatase titania, or a combination thereof. Anexemplary commercially available titania source suitable for use in theinstant invention is the Ti-pure titania, available from Dupont,Wilmington, Del., USA. Titania sources are not limited to the rutilestructure. Anatase sources which are usually of finer particles sizesmay also be use. When anatase is used as the source of the titania, thetitania source may be spray-dry to aggregate the particles to particlesize greater than 1 micron, or even 5-20 microns aggregates. Rutiletitania particle range may be between 1-25 microns, or even between10-20 microns.

The zirconia source can in one aspect be a zirconium oxide (ZrO₂). Tothat end, an exemplary commercially available zirconia source suitablefor use in the instant invention is the Z19 zirconia, available fromZircoa, Inc., Solon, Ohio USA. It is preferable that the zirconia medianparticle size is at least 2 microns.

Yttrium phosphate (YPO₄) is a naturally occurring mineral, also referredto as Xenotime. A yttrium phosphate or xenotime powder can also beobtained by reacting yttrium oxide and phosphorous oxide under a varietyof conditions effective to provide yttrium phosphate. In an exemplaryprocess, a mixture of yttrium oxide and phosphorous oxide can becalcined in a platinum crucible at 190° C. for approximately 24 hours.The calcined mixture can then be reacted at an elevated temperature foranother 24 hours. Following the reaction, the resulting yttriumphosphate can be ground to a desired particle size using, for example,alumina media and sieving to a desired mesh size. Optionally, theyttrium source may be yttrium oxide and the phosphate source may bephosphorous oxide.

One of ordinary skill in the art will appreciate that the amount of theabove-described inorganic batch components can be selected to yield aresulting ceramic composition having the desired oxide components anddesired weight fractions as described herein. To that end, the selectionand/or determination of such amounts will be readily obtainable by oneof ordinary skill in the art in view of the instant disclosure andwithout requiring undue experimentation and thus will not be discussedin detail herein. However, in exemplary aspect, the amount of aluminumtitanate is estimated to be between 50-90% by weight, and in someembodiments, even between 60-80% by weight. Similarly, the secondaryphase (zirconium titanate or Yttrium phosphate) is estimated to bebetween 10-50% by weight, and in some embodiments, between 20-40%.Traces of corundum may be observed. It one aspect, there is no titanialeft which is unreacted following sintering, or only a trace is leftunreacted, i.e., less than 0.05 wt. %. In the absence of a sinteringaid, substantial amounts of zirconia, titania, and alumina are presentwhen fired below 1500° C.

As indicated above, the inorganic ceramic forming batch composition canfurther comprise a sintering aid. The addition of the sintering aid can,for example, enhance the strength of the ceramic composition afterfiring. Suitable sintering aids generally include an oxide source of oneor more metals such as strontium, barium, iron, magnesium, zinc,calcium, aluminum, lanthanum, yttrium, titanium, bismuth, molybdenum ortungsten. In one aspect, and as described herein, it is preferred thatthe sintering aid comprises yttrium oxide.

The plasticized ceramic precursor batch composition can further comprisean optional pore forming agent in order to optimize the porosity andpore size distribution in the fired body for a particular application.To this end, a pore former is a fugitive material which can, forexample, evaporate or undergoes vaporization by combustion during dryingor heating of the green body to obtain a desired, usually higherporosity and/or coarser median pore diameter. A suitable pore former caninclude, without limitation, carbon; graphite; starch; wood, shell, ornut flour; polymers such as polyethylene beads; waxes; and the like. Thepore former can be utilized in virtually any amount sufficient toachieve desired levels of porosity. For example, the plasticized ceramicprecursor batch composition can comprise one or more pore formers in anamount in the range of from 0 weight percent up to 70 weight percent,including exemplary amounts of 20 weight %, 30 weight %, 40 weight % oreven 50 weight %.

The inorganic batch components and any optional pore former componentcan be intimately blended with a liquid vehicle and forming aids whichimpart plastic formability and green strength to the raw materials whenthey are shaped into a green body. Exemplary forming aids can include,without limitation, plasticizers, surfactants, lubricants, and temporarybinders.

Forming of the green body may be done by, for example, molding orextrusion. When forming is done by extrusion, most typically a celluloseether temporary binder such as methylcellulose, hydroxypropylmethylcellulose, methylcellulose derivatives, and/or any combinationsthereof, serve as a binder. Exemplary lubricants can also include ahydrocarbon oil, tall oil, oleic acid, sodium stearate, tri-ethanolamine, or any combination thereof. Still further, the liquid vehiclecomponent can in an exemplary aspect comprise water. In another aspect,depending on the component parts of the ceramic batch composition, itshould be understood that organic solvents such as, for example,methanol, ethanol, or a mixture thereof can also be used as the liquidvehicle.

The relative amounts of the forming aids can vary depending on factorssuch as the nature and amounts of raw materials used, etc. For example,in one aspect, the organic binder is present in as a super addition inan amount in the range of from about 2% to about 10% by weight of theplasticized composition. Similarly, the liquid vehicle component canvary depending on the type of material used in order to impart optimumhandling properties and compatibility with the other components in theceramic batch mixture. For example, in one aspect, the liquid vehiclecan be present as a super addition in an amount in the range of from 15%to 60% by weight of the plasticized composition, and more preferably inthe range of from 20% to 50% by weight of the plasticized composition.

Once intimately blended, the resulting stiff, uniform, and extrudableplasticized ceramic precursor batch composition can then be shaped intoa green body by any known conventional ceramic forming process, such as,e.g., extrusion, injection molding, slip casting, centrifugal casting,pressure casting, dry pressing, and the like. In an exemplary aspect,extrusion can be done using a hydraulic ram extrusion press, or a twostage de-airing single auger extruder, or a twin screw mixer with a dieassembly attached to the discharge end. In the latter, the proper screwelements are chosen according to material and other process conditionsin order to build up sufficient pressure to force the batch materialthrough the die.

The instant method and the resulting ceramic structures are in oneaspect especially suited for use as diesel particulate filters and, inparticular, for adsorbing one or more of Hydrocarbons (HC), CarbonMonoxide (CO), Nitrogen Oxides (NOx), and Particulate Matter (PM) from astream of combustion exhaust gas. To that end, the inventive ceramicbodies are especially suited as multi-cellular honeycomb structureshaving a high filter volumetric heat capacity, a low pressure dropbetween the entrance and exit faces of the filter, a low CTE, and highfiltration efficiency. To this end, in one aspect the plasticizedceramic precursor batch composition can be formed or otherwise shapedinto a honeycomb configuration.

Although a honeycomb ceramic filter of the present invention normallyhas a structure in which a plurality of through holes opened to the endsurface of the exhaust gas flow-in side and to the end surface of theexhaust gas flow-out side are alternately sealed at both the endsurfaces, the shape of the honeycomb filter is not particularlyrestricted. For example, the filter may be a cylinder having endsurfaces with a shape of a circle or an ellipse, a prism having the endsurfaces with a shape of a polygon such as a triangle or a square, ashape in which the sides of these cylinder and prism are bent like an“doglegged shape,” or the like. In addition, the shape of through holesis not particularly limited. For example, the sectional shape may be apolygon such as a square or an octagon, a circle, an ellipse, or thelike.

Some examples of ceramic honeycombs produced by the process of thepresent invention generally comprise cell densities in the of range from235 cells/cm² (about 1500 cells/in²) to 15 cells/cm² (about 100cells/in²), including those having about 94 cells/cm² (about 600cells/in²), about 62 cells/cm² (about 400 cells/in²), or about 47cells/cm² (about 300 cells/in²), those having about 31 cells/cm² (about200 cells/in²), or those having about 15 cells/cm² (about 100cells/in²). Further, the wall or web thicknesses typically range fromabout 0.07 to about 0.6 mm (about 3 to about 25 mils). It should howeverbe understood that the particular desired size and shape of the ceramicbody can depend on the application, e.g., in automotive applications byengine size and space available for mounting, etc. To this end, althoughthe ceramic articles of the instant invention are, in one aspect,suitable for preparing thin-walled honeycombs, the claimed mixtures canalso be used for thicker walled structures. For example, honeycombsstructures having about 15 to about 30 cells/cm² (about 100 to about 200cells/in²) and about 0.30 to about 0.64 mm (about 12 to about 25 mil)wall thicknesses are well suited for diesel particulate filterapplications.

The formed green body having a desired size and shape as described abovecan then be dried to remove excess moisture. The drying step can beperformed by hot air, microwave, steam, or dielectric drying, which canbe followed by ambient air drying. Once dried, the green body canthereafter be fired under conditions effective to convert the green bodyinto a ceramic article comprising a primary crystalline phase ceramiccomposition as described below.

The firing conditions effective to convert the green body into a ceramicarticle can vary depending on the process conditions such as, forexample, the specific composition, size of the green body, and nature ofthe equipment used. To that end, in one aspect the optimal firingconditions needed to obtain a ceramic article according to the instantinvention will be obtainable by one of ordinary skill in the art throughno more than mere routine experimentation. Thus, the present inventionis not limited to specific firing temperatures and times. However, inone aspect, the firing temperatures can range from about 1400° C. toabout 1600° C. In another aspect, the firing temperature can range fromabout 1450° C. to about 1550° C. The firing times can also range fromabout 1 hour to about 6 hours, including exemplary firing times of 2, 3,4, or 5 hours.

Once the green body has been fired, a catalyst washcoating can beapplied to the resulting porous ceramic article. In one aspect, thewashcoating can be an alkali washcoating solution such as, for example,an alumina/barium, or alumina/potassium based washcoating solution.Unless otherwise specified, the typical potassium washcoat solution usedis prepared by mixing potassium nitrate (Sigma Aldrich, St. Louis, Mo.)with gamma alumina (Grace Chemicals, Columbia, Md.) in 10/100 w/w ratio.The resulting mixture is then dried and calcined at 550° C. for threehours. The resulting powder is milled and to that a15% weightcommercially available alumina based washcoating and binder solutionNyacol® Al20 is added, Nyacol® Al20 colloidal alumina sol iscommercially available from Nyacol Nano Technologies, Inc., Ashland,Mass., U.S.A. For deep bed wall flow (filter) test, the gamma aluminaand Nyacol® Al20 is mixed and washcoated initially followed byintroduction of the potassium nitrate solution Prior to applying thecatalyst washcoating, a pre-coating or passivation step can be used toimprove the properties of the catalyzed substrates, by reducing catalystand/or support coating diffusion into the relatively fine pore andmicrocrack structure of the formed ceramic article. Exemplarypre-coating or passivation steps can include one or more of thosedisclosed and described in United States Patent Application PublicationNo.: 2005/0037147, the entire disclosure of which is incorporated byreference herein for all purposes.

The washcoating can be applied by any conventionally known method. Forexample, the washcoating process can comprise dip-coating the porousceramic article in a washcoating solution for a period of timesufficient to achieve a substantially even coating layer. Excess coatingcan then be removed from the sample using, for example, compressed air.The washcoated ceramic article can also be dried to provide theresulting catalyst-coated or washcoated ceramic article.

EXAMPLES

To further illustrate the principles of the present invention, thefollowing examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how theceramic articles and methods claimed herein can be made and evaluated.They are intended to be purely exemplary of the invention and are notintended to limit the scope of what the inventors regard as theirinvention. Efforts have been made to ensure accuracy with respect tonumbers (e.g., amounts, temperatures, etc.); however, some errors anddeviations may have occurred. Unless indicated otherwise, parts areparts by weight, temperature is degrees C. or is at ambient temperature,and pressure is at or near atmospheric.

Examples 1-4

Four inventive sintered phase ceramic oxide compositions consistingessentially of zirconium titanate and aluminum titanate (ZT-AT), invarying relative oxide weight fractions, were prepared from theexemplary batch compositions set forth in Table 1 below.

TABLE 1 Batch Compositions for Examples 1-4 Batch Composition A B C %Aluminum Titanate (AT) 80 70 60 % Zirconium Titanate (ZT) 20 30 40Inorganic Batch Materials Weight % Weight % Weight % Zirconia (Z19-Zircoa) 12.12 18.18 24.17 Al₂O₃ (A10 - Alcoa) 44.86 39.25 33.64 TiO2(Ti-Pure - Dupont) 43.02 42.57 42.19 Y₂O₃ (H. C. Starck) 3 3 3 Additivesand Forming Aids Weight % Weight % Weight % Pore Former (A625 - Asbury)30 30 30 Methocel F241 4.5 4.5 4.5 Emulsion* 16 16 16 *An emulsioncomprising 6.25% Tall oil in water with 0.94% of 97% tri-ethanol amine

As indicated, the relative weight percentages of the inorganic batchmaterials were varied among the three batch compositions in order toprovide resulting ceramic articles having aluminum titanate andzirconium titanate in varying relative weight fractions. In particular,the batch compositions were adjusted to provide sintered phase ceramicarticles having aluminum titanate weight fractions of 80%, 70% and 60%respectively. Likewise, the relative oxide weight fractions of thezirconium titanate in the resulting ceramic articles were 20%, 30% and40% respectively.

The ceramic articles can be prepared by pre-sizing and turbula mixingthe powdered materials. The batch additives and forming aids, includingthe liquid vehicle, organic binder, and any lubricant can be added tothe resulting powder mixture and subsequently mulled together. Theresulting material can then extruded, and the resulting cellular warescan be dried by, for example, a humidity controlled oven or microwave.For Examples 1-3, three two inch extruded green bodies, each comprisingone of the plasticized batch composition A, B, and C, of Table 1 werefired at 1500° C. for approximately 6 hours. Example 4 was similarlyprepared by firing a fourth extruded 2 inch green body, also comprisingthe plasticized batch composition “C” at approximately 1475° C. forapproximately 6 hours. Except where noted, testing of the four resultingceramic articles was performed on cellular wares having 200 cells persquare inch and 12 mil wall thicknesses.

The four ceramic articles were then evaluated for several physicalproperties, including coefficient of thermal expansion, total porosity,median pore size and strength. The results of the evaluation are setforth in Table 2 below.

TABLE 2 Measured Physical properties of Examples 1-4 Example # 1 2 3 4Batch Composition A B C C Firing Temperature/ 1500° C./ 1500° C./ 1500°C./ 1475° C./ Time 6 hr 6 hr 6 hr 6 hr CTE (×10⁻⁷/° C.) 7.2 5.4 4.1 7.4Porosity (%) 53 52 45 50 MPS (μm) 13 12 20.8 17 Strength (psi - rods)503 788 — 725

A sample ceramic article prepared according to the procedures set forthfor Example 1, comprising 80% AT/20% ZT, was subsequently evaluated forits compatibility with a potassium containing washcoat. The washcoat wasapplied primarly on the surface of the ceramic web in order to simulatea typical coating condition used for automotive substrates. FIG. 1 showsthe electron microprobe result of the potassium washcoat on the 80%AT/20% ZT sample after aging at 850° C. for 10 hours in 10% steam. Asshown, no potassium reaction or leaching was observed in theinterstitial microstructure for up to 6 wt % K₂O after the acceleratedaging test.

A second sample of a ceramic article prepared according to theprocedures set forth for Example 1, comprising 80% AT/20% ZT, was againevaluated for its compatibility with a potassium containing washcoat. Inthis case a 0.6 M potassium washcoat was applied in the pores of thewalls as would typically be the case in a 4 way adsorber application.FIG. 2 shows the electron microprobe result of the potassium washcoat onthe 80% AT/20% ZT sample after aging at 850° C. fcr 10 hours in 10%steam environment. Again, no significant reaction was observed in theanalyzed using electron microprobe upto 3.5% wt K₂O after acceleratedaging.

A sample ceramic composition prepared according to Example 2, comprising70% AT/30% ZT, was evaluated to determine the stability of the aluminumtitanate phase in the inventive ceramic composition. As shown in FIG. 3,no decomposition or growth was observed after a 100 hour hold period at800° C. Although a relatively small amount of decomposition was observedafter 100 hours of testing at 1000° C., the results of this testindicate that the exemplified composition can in one aspect be used attemperatures of up to 1000° C. for limited periods without resultingdecomposition or growth.

A sample prepared according to Example 2 comprising of 70% AT/30% ZT waspre-passivated with ionene according to U.S. Pat. No. 7,132,150. Theinventive sample was washcoated with 0.9% weight of the potassiumnitrate/gamma alumina/Al-20 washcoat slurry and aged under 10% steam at800° C. for 15 hours. The post measured coefficient of thermal expansionup to 1000° C. decreased by 1.6 points.

A sample inventive ceramic composition prepared according to theprocedures set forth for Example 1, comprising 80% AT/20% ZT, was alsoevaluated for its pressure drop under a range of soot loadings. FIG. 4provides the pressure drop curve obtained for the 80% AT and 20% ZTsample. For a typical “4 way” adsorber application, the targeted sootloading per regeneration is 1-2 g/l as indicated in the plot. The datareported in FIG. 4 indicates that, when compared with targeted pressuredrop values reported in FIG. 5, the pressure drop value of the inventiveceramic article at 2 g/l soot loading is comparable to that ofconventional articles having 60% or higher levels of porosity.

Still further, FIG. 6 is an exemplary scanning electron micrograph ofthe 80% AT/20% ZT sample prepared according to Example 1. As shown, thewell connected pores of the microstructures are visible with the whitegrains being the zirconium titanate phase and the gray being thealuminum titanate phase.

Examples 5-13

The suitability of yttrium phosphate as a non-silicate additive andstrengthener for an aluminum titanate ceramic article according to thepresent invention was also evaluated with three extrusion batches. Tothat end, three inventive sintered phase ceramic oxide compositionscomprising yttrium phosphate and aluminum titanate (YP-AT) were preparedfrom the exemplary batch compositions set forth in Table 3 below.

TABLE 3 Batch Compositions used for Examples 5-13 Batch Composition D EF Weight % Weight % Weight % Inorganic Batch Materials Al₂O₃ (A10 -Alcoa) 39.1 39.1 39.1 TiO₂ - RMA 30.7 30.7 30.7 Yttrium Phosphate - A30.2 — — Yttrium Phosphate - B — 30.2 — Yttrium Phosphate - C — — 30.2Additives and Forming Aids Pore Former (A625 - Asbury) 30 30 30 MethocelA4M 4.5 4.5 4.5 Oleic Acid 1.0 1.0 1.0 Water As needed As needed Asneeded

As indicated in Table 3, the three extrusion batch compositions areequivalent except for the yttrium phosphate component. The three yttriumphosphate batch materials were prepared by first turbula mixing powdersof yttrium oxide and phosphorous oxide for 1 hour, then calcining themixture in a platinum crucible at 190° C. for 24 hours. The calcinedmixtures were then reacted at an elevated temperature for 24 hours. Thereaction products were then ground with alumina media and sieved to −325mesh size. Yttrium phosphate batch code “A” was reacted at 1200° C. andcontained a slight excess of phosphorous oxide in the reaction mixture.Yttrium phosphate batch code “B” was reacted at 1475° C. and thereaction mixture contained approximately 0.5 weight percent of excessyttrium oxide. Yttrium phosphate batch code “C” was reacted at 1200° C.and the reaction mixture contained stoichiometric amounts within thelimits of the weighing error.

Plasticized extrusion batches were prepared by turbula mixing theinorganic powder batch materials for approximately 15 minutes, followedby mulling for the addition of the additives and forming aids. Two dieswere used for extrusion. A rod was used for measurement of modulus ofrupture and was produced with a ¼ diameter die. A one inch diameterhoneycomb die was used to make cellular wares having 200 cells persquare inch and 16 mil thick wall for expansion and porositymeasurements. The extruded wares were dried in a humidity controlledoven for 72 hours. Samples for the three batches were then fired atthree sintering temperatures, 1400, 1500, and 1600° C., forapproximately 4 hours. The modulus of rupture (MOR), coefficient ofthermal expansion (CTE), and porosity for each sample were determined.The results are summarized and reported in Table 4 below.

TABLE 4 Measured Physical Properties of Examples 5-13 Ex. # 5 6 7 8 9 1011 12 13 Extrusion Batch D D D E E E F F F Firing (° C.) 1400 1500 16001400 1500 1600 1400 1500 1600 MOR (psi) 488 1036 2161 522 609 857 536617 843 CTE (×10⁻⁷/° C.) 28 12 — 18 15 5.6 21 12 10.5 MPS (μm) 7 10 1.39 8 11 9 10 11 Porosity (%) 56 46 18 54 41 41 56 49 41

SEM micrographs of inventive Examples 6 and 12, are shown in FIGS. 7 and8, respectively. As illustrated, grains with bright white contrastrepresent the yttrium phosphate and the gray phase represents aluminumtitanate. Both the pore microstructure and the aluminum titanate phaseare realtively well-connected and continuous.

Lastly, it should also be understood that while the present inventionhas been described in detail with respect to certain illustrative andspecific embodiments thereof, it should not be considered limited tosuch, as numerous modifications are possible without departing from thebroad spirit and scope of the present invention as defined in theclaims.

1. A ceramic article, comprising a sintered phase ceramic compositioncontaining, as expressed on a weight percent oxide basis:a(Al₂TiO₅)+b(ZrTiO₄)+c(Y₂O₃)+d(YPO₄) wherein a, b, c, and d are weightfractions of each component such that (a+b+c+d)=1.00, and wherein0.5<a≦0.95; 0≦b≦0.5, 0.0≦c≦0.10, and 0≦d≦0.5.
 2. The ceramic article ofclaim 1, wherein the sintered phase ceramic composition consistsessentially of aluminum titanate, zirconium titanate, and yttrium oxide,and wherein 0.60≦a≦0.80 and 0.20≦b≦0.40 and 0.00≦c≦0.05.
 3. The ceramicarticle of claim 1, wherein the sintered phase ceramic compositionconsists essentially of aluminum titanate and yttrium phosphate, wherein0.5≦a≦0.95 and 0.05≦d≦0.5.
 4. The ceramic article of claim 1, whereinthe sintered phase ceramic composition has a total porosity greater thanor equal to 45%.
 5. The ceramic article of claim 4, wherein the sinteredphase ceramic composition has a total porosity greater than or equal to50%.
 6. The ceramic article of claim 4, wherein the sintered phaseceramic composition has a total porosity in the range of from 50% to60%.
 7. The ceramic article of claim 1, wherein the sintered phaseceramic composition has a coefficient of thermal expansion in at leastone direction that is less than or equal to 15.0×10⁻⁷/° C. across thetemperature range of from 25° C. to 1000° C.
 8. The ceramic article ofclaim 1, wherein the sintered phase ceramic composition has acoefficient of thermal expansion in at least one direction that is lessthan or equal to 12.0×10⁻⁷/° C. across the temperature range of from 25°C. to 1000° C.
 9. The ceramic article of claim 1, wherein the sinteredphase ceramic composition has a coefficient of thermal expansion in atleast one direction that is less than or equal to 10.0×10⁻⁷/° C. acrossthe temperature range of from 25° C. to 1000° C.
 10. The ceramic articleof claim 1, wherein the sintered phase ceramic composition has acoefficient of thermal expansion in at least one direction that is lessthan or equal to 20.0×10⁻⁷/° C. across the temperature range of from 25°C. to 1000° C. after aging in steam at 850° C. for 10 hours.
 11. Theceramic article of claim 1, further comprising an alkali catalystwashcoating.
 12. The ceramic article of claim 1, wherein the alkalicatalyst washcoating comprising potassium.
 13. The ceramic article ofclaim 1, wherein the ceramic article is a honeycomb body.
 14. A methodfor producing a ceramic article, comprising the steps of: providing aplasticized ceramic precursor batch composition containing an aluminasource, a titania source, and at least one of a zirconia source, ayttria source, a phosphate source, and a yttrium phosphate source; anorganic binder; and a liquid vehicle; forming an extruded green bodyfrom the plasticized ceramic precursor batch composition; and firing theformed green body under conditions effective to convert the ceramicprecursor composition into a sintered phase ceramic composition.
 15. Themethod of claim 14, wherein the inorganic batch composition comprises analumina source, a titania source, and a zirconia source.
 16. The methodof claim 15, wherein the inorganic batch composition further comprisesan yttrium oxide sintering aid.
 17. The method of claim 14, wherein theinorganic batch composition comprises an alumina source, a titaniasource, and a yttrium phosphate source.
 18. The method of claim 14,further comprising the step of drying the formed green body prior tofiring the green body.
 19. The method of claim 14, wherein the firingconditions effective to convert the ceramic precursor composition into asintered phase ceramic composition comprise firing the formed green bodyat a temperature in the range of from 1400° C. to 1600° C.
 20. Themethod of claim 19, wherein the firing conditions effective to convertthe ceramic precursor composition into a sintered phase ceramiccomposition comprise firing the formed green body at a temperature inthe range of from 1450° C. to 1550° C.
 21. The method of claim 18,wherein after firing substantially no titania remains unreacted.
 22. Themethod of claim 14, wherein the extruded green body is a honeycomb body.